feat: fixup language around explicit valid item

Author: gadomski

README.md

@@ -204,7 +204,10 @@ JSON
 
 ### Explicitly get a valid STAC Item
 
-This has a similar effect as an empty object for `fields`, but it is up to the discretion of the implementation.
+Because implementations may choose to always include other fields (e.g.,
+extension-specific fields such as
+[sar](https://github.com/stac-extensions/sar)), this could has the same effect
+as an empty object for `fields`.
 
 Query Parameters